Interferometry has a number of useful applications including the measuring of displacements of various structures. For example, in the field of thin film mechanics, the well-known Michelson interferometer is often used in connection with bulge and blister testing, which are two conventional tests used to determine various material properties of thin films, such a Young's modulus, rupture strength, Poisson's ratio and adhesion, among others. In bulge testing, a test specimen may be created by first depositing a film to be tested onto a substrate, which is typically a silicon wafer having a silicon nitride layer on each of its front and back surfaces. Then, a window is created through the substrate by removing portions of the substrate using known masking and etching techniques.
To perform the bulge test, the specimen is placed onto a pressurization mount for applying a positive (or negative) pressure, relative to the ambient pressure, to the window. As the pressure is increased, the film window bulges in a direction away from the substrate, causing the film to displace by various amounts over the region of the window. By measuring the displacement of the film window and the pressure causing that displacement, and knowing the behavior of membranes under such loading conditions, several properties of the particular film may be calculated. Blister testing also utilizes a thin film window. However, instead of causing the film window to bulge only at the region of the window, the bulge is further pressurized to cause the film to delaminate from the substrate to determine the adhesion properties of the film.
Since the films tested using bulge and blister testing procedures are often very thin, e.g., on the order of 500 nm to 1,000 nm, and the corresponding displacements are also small, typically on the order of 10 μm to 100 μm, very precise displacement measurements are required. One known method of making such small, precise measurements is to use a Michelson interferometer in conjunction with a monochromatic laser light source. As is commonly known in the art, a properly-calibrated Michelson interferometer used for measuring the displacement of a bulging window produces an interference pattern consisting of alternating rings of fringes, i.e., regions of relatively intense light, and nodes, i.e., regions of relatively little or no light. The interference pattern is caused by constructive and destructive combination of a reference beam of the laser light reflected from a stationary mirror with a measuring beam of the laser light reflected from the bulge. As the bulge continues to displace away from the substrate, the fringes and nodes move outward from the center of the bulge as the measuring beam continuously shifts relative to the reference beam. Thus, to determine the displacement of the bulge between any two points in time, or pressures, the number of fringes (or nodes) passing a fixed point between those two times, or pressures, needs to be counted. The displacement may then be calculated by multiplying the number of fringes by one-half the wavelength of the light from the laser.
Presently, fringe counting is performed either manually, i.e., by projecting the interference pattern onto a screen and an observer viewing the screen and counting the number of fringes that pass a fixed reference point on the screen, or automatically, e.g., using a photodetector aimed at a fixed reference point to detect the alternating light intensities at the fixed reference point as the fringes pass by while the bulge is being inflated (or deflated). In conventional automated fringe counting systems, the photodetector is linked to a computer configured to graphically display the light intensity, time and/or pressure data on a computer screen.
These conventional fringe counting methods have an number of shortcoming. For example, in manual counting, it can be difficult for the observer to concentrate and remain focused on a fixed point throughout the entire test. Often, two people are used to manually count the fringes. One person observes the pattern and calls out when a new fringe passes the fixed point, and the other person records the passing of the new fringe. Manual counting is plagued by its relatively high potential for human error. Automated fringe counting eliminates many of the problems of manual counting, but the use of a photodetector creates some shortcomings of its own. For example, it can take a significant amount of time to properly aim the photodetector at the desire sampling point. In addition, typically a single photodetector is used to detect fringes at only a single sampling point. This, does not allow for redundancy and/or the detection of phenomena other than the passing of fringes past the sole reference point.
Other interferometry techniques may be used to measure the displacement of the bulge during bulge or blister testing. For example, instead of using a monochromatic laser, a white light diode may be used. In white light interferometry, the interference pattern generated by the interferometer is most intense when the distance the reference beam travels is equal to the distance the measuring beam travels. Thus, to determine displacement of the film at the bulge, it is necessary to move either the test specimen or the mirror reflecting the reference beam as the film displaces to make the travel lengths of the reference and measuring beams equal to one another. This requires the use of a precision movable stage and a corresponding control system that can add significantly to the cost and complexity of the bulge/blister testing apparatus.